Polyesters are used in various fields including fibers, molded parts, and films because they are excellent in easiness of molding processing, mechanical properties, heat resistance, chemical resistance, flavor retention, and other physical or chemical characteristics.
Above all, polybutylene terephthalate comprising a terephthalic acid component and a 1,4-butanediol component is widely used in injection molded articles such as automotive parts, electric or electronic parts, and precision machine parts as one of engineering plastics having mechanical properties and heat resistance, which can be a substitute for metallic materials, and in recent years, has also been widely used in fields including films, sheets, monofilaments, and fibers because of its excellent properties.
In general, polyesters are prepared by esterification reaction or ester exchange reaction between a dicarboxylic acid or an ester derivative thereof and a diol and subsequent polycondensation reaction usually under a reduced pressure of 10 kPa or less while removing formed water and low-molecular weight materials such as diols out the system.
At this time, for the sake of reducing the pressure of a polycondensation reaction vessel, a deaerator is necessary, and for example, various types such as liquid ring pumps, oil rotary pumps, Roots pumps, and steam ejectors are known. A method of designing to maximize the performance by combining a plurality of them is frequently carried out.
Above all, a combination of a steam ejector, a condenser placed in the downstream portion of the ejector, and a hot well tank connected to the condenser via a barometric leg is widely used because the apparatus is simple, mechanical driving portions that are liable to cause troubles are not provided, and a desired high degree of vacuum is obtained (for example, see Patent Document 1).
However, the steam ejector utilizing steam involved a problem such that organic components contained in a sucked gas are collected in the condenser and condensed in sealing water in the hot well tank and in its terns introduce an increase of COD of waste water, whereby a load against the environment increases.
For solving this problem, there is also known a method in which in polyesters such as polyethylene terephthalate, a combination of an ejector using a vapor of a starting material diol (ethylene glycol in the case of polyethylene terephthalate) in place of steam, a condenser placed in the downstream portion of the ejector, and a hot well tank connected to the condenser via a barometric leg is used. By using this method, organic components contained in a sucked gas can be condensed in the diol as a sealing liquid of the condenser, and by recycling this liquid as the starting material, polyesters can be prepared without discharging a waste liquid with high COD outside the system (for example, see Non-Patent Document 1).
On the other hand, in the case of using 1,4-butanediol as a driving vapor of the ejector, there was involved a problem such that tetrahydrofuran (THF) or H2O is generated according to heat history of the vapor generation of the ejector; and in a plurality of condensers having a different performance as placed in the downstream portion of the ejector, gas-liquid distribution takes place corresponding to pressures of the respective condensers, and THF, H2O or the like condensed in the condenser having a high pressure increases a vapor pressure of THF, H2O or the like in the condenser having a low pressure via the hot well tank and ultimately exceeds a critical backing pressure of the ejector using the condenser having a low pressure, whereby a prescribed pressure is not obtained.
Also, from the viewpoint of recycling into the starting material, in many cases, in the case of preparing polyesters using 1,4-butanediol as one monomer component, an ejector using 1,4-butadiol as a driving vapor is used. In this case, there was also involved a problem such that similar to the foregoing, low-boiling components formed from the polycondensation reaction vessel, such as THF and water, increase the vapor pressure of the sealing liquid of the hot well tank and ultimately exceeds a critical backing pressure of the ejector, whereby a prescribed pressure is not obtained.
[Patent Document 1]
JP-A-57-135828
[Non-Patent Document 1]
Internet
http://www.koerting.de/S/prd/en/02/06/01.html
The invention has been made for the purpose of providing a process capable of realizing a stable reduced pressure state and producing polyesters having a stable quality.